EP1243893A1

EP1243893A1 - A crystal based fringe generator system

Info

Publication number: EP1243893A1
Application number: EP02100183A
Authority: EP
Inventors: Chen Fang; James Stewart Ii Rankin; Mumin Song; Paul Joseph Stewart
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2001-03-20
Filing date: 2002-02-22
Publication date: 2002-09-25
Also published as: US6760113B2; US20020135773A1

Abstract

A measurement system 10 for accurately measuring the surface geometry of a part 32 in three dimensions includes a laser 12 for emitting a laser beam 14. The laser beam 14 is transmitted to a birefringent crystal 16 which splits the laser beam 14 into a pair of beams 18,20. The pair of beams 18,20 are then subjected to a phase shift by a liquid crystal system 24 as controlled by a computer 26. The pair of beams 18,20are then expended in order to form a fringe pattern 30 on the surface of the part 32 to be measured.

Description

The present invention relates generally to an optical three-dimensional measurement system and in particular to a non-contact optical three dimensional measurement system using a laser structured light generator.
Many current systems exist for measuring the geometry of three-dimensional surfaces. One such system is a coordinate measurement system (CMM) which is used to measure die stamps, stamped panels, and other vehicle body structures.
The current CMM systems suffer from a variety of disadvantages. First, these CMM systems are relatively slow which limits the number of measurements that they can take during any given period of time. Second, these CMM systems require surface contact in order to function properly, which can potentially damage the surface of the part being measured.
Additionally, depending upon the geometry of the surface being measured, an instrument that needs to contact the surface to perform measuring risks damage due to contact with the surface as it moves to various surface points.
These CMM systems are thus typically only used in connection with small-scale projects and this makes them relatively inefficient.
Additionally, current measurement systems typically have a variety of mechanically moving parts. These moving parts can cause vibrations in the system, which can affect the accuracy of the measurements. In order to reduce the inaccuracy and/or unreliability in the measurements of the systems that utilize mechanically moving parts, the system has to allow time for the vibration to settle before the measurements are taken. This, however, adds to the time of the process and therefore increases the cost. These systems also have limitations in data acquisition speed, size, and reliability.
U.S. Patent No. 6,100,984 discloses a system that solves many of the above-noted problems and improves on prior measurement systems.
The Patent discloses a measurement system including a laser which emits a beam through an objective lens, which expands the laser beam into a diverged beam. The diverged beam passes through a liquid crystal system that is located forwardly of the objective lens with respect to the laser. The liquid crystal system receives the diverged beam and generates at least one fringe pattern on a surface of a part to be measured. The liquid crystal system is in communication with a computer to control the pitch and phase of the fringe pattern.
While the system disclosed in U.S. Patent No. 6,100,984 has been found to be extremely effective, it would be advantageous if a measurement system could be developed for measuring part surface or other surface geometry that was more compact, more accurate, and more efficient.
It is an object of the present invention to provide a non-contact optical based three dimensional measurement system that is light in weight, compact, and accurate.
According to a first aspect of the invention there is provided a fringe generator for use in connection with a measurement system for measuring the surface geometry of a part, comprising a laser for emitting a laser beam a transparent device disposed forwardly of said laser for splitting said laser beam into a plurality of beams, a liquid crystal system disposed forwardly of said crystal, a computer in communication with said liquid crystal system to perform phase shifting on said plurality of beams and a beam expander for expanding said pair of beams to form a fringe pattern on the part.
The liquid crystal system may be a liquid crystal polarization based phase shifter.
The system may further comprise a polarizer which is disposed between said transparent device and said liquid crystal system.
Alternatively, the liquid crystal system may be a liquid crystal based phase retardation system.
The system may further comprise a polarizer which is disposed between said liquid crystal system and said beam expander.
Preferably, said transparent device and said liquid crystal system are combined into a single structure.
As yet another alternative the liquid crystal system may be a spatial light modulator.
Preferably, said transparent device may be a birefringent crystal.
According to a second aspect of the invention there is provided a method for generating a fringe on a part surface, which is to be measured, comprising emitting a beam from a light source, splitting said beam into a plurality of beams, phase shifting said plurality of beams and expanding said pair of beams such that they overlap in space to interfere with each other to form a fringe pattern on the surface of the part.
The method may further comprise orienting the polarization directions of each of said plurality of beams in the same direction.
Said orienting may be performed before said phase shifting of said plurality of beams.
Alternatively, said orienting may be performed after said phase shifting of said pair of beams.
Preferably, said phase shifting may be controlled by a computer system.
Advantageously, said splitting and said phase shifting are performed by a single structure.
According to a third aspect of the invention there is provided a method for accurately measuring without contact the surface of a vehicle body part utilising a method in accordance with the second aspect of the invention.
According to a fourth aspect of the invention there is provided a fringe generator for measurement system for accurately measuring the surface geometry of a part, the fringe generator comprising a laser device for emitting a laser beam, a birefringent crystal located so as to receive said laser beam and for splitting said laser beam into a plurality of beams a computer for controlling phase shifting of said plurality of beams and a polarizer for orienting the polarization directions of said pair of beams in the same direction.
The system may further comprise a liquid crystal system disposed forwardly of said birefringent crystal and in communication with said computer for performing said phase shifting.
The system may further comprise a beam expander for expanding said plurality of laser beams to form at least one fringe pattern.
Said polarizer may be located between said birefringent crystal and said liquid crystal system.
Alternatively, said polarizer is located between said liquid crystal system and said beam expander.
Preferably, said birefringent crystal may be in communication with said computer to perform said phase shifting.
The invention will now be described by way of example with reference to the accompanying drawing of which:-
Figure 1 is a schematic illustration of a liquid crystal polarization and birefringent crystal based fringe generation in accordance with a preferred embodiment of the present invention;
Figure 2 is a schematic illustration of liquid crystal retardation and birefringent crystal based fringe generator in accordance with a preferred embodiment of the present invention;
Figure 3 is a schematic illustration of a liquid crystal retardation based fringe generator in accordance with a preferred embodiment of the present invention; and
Figure 4 is a schematic illustration of a liquid crystal two-source fringe generator in accordance with a preferred embodiment of the present invention.
Referring now to Figures 1 through 4, which illustrate various embodiments of a light source 10 of a measurement system in accordance with the present invention, the disclosed measurement system is a non-contact optical three-dimensional measurement system for measuring the three dimensional geometry of various surfaces, including dies, stamping panels, vehicle body structures as well as a variety of other structures in both automotive and non-automotive applications.
The light source 10 generates a grid or other pattern on the surface to be measured.
With particular reference to Figure 1, the light source 10 includes a laser 12, which emits a laser beam, generally indicated by reference number 14. The laser beam 14 is directed to a birefringent crystal 16. While a birefringent crystal is preferably utilized, any transparent light divider, such as a prism may be utilized.
The birefringent crystal 16 splits the laser beam 14 into two separate beams, generally indicated by reference numbers 18 and 20. It should be understood that the beams 14 may be divided or subdivided into more beams as desired.
The separate beams 18, 20 are then preferably passed to a circular polarizer 22 which is used to equalise the intensity of the two beams 18, 20.
While a circular polarizer 22 is preferably used a variety of other polarizers or similar structures may be used for accomplishing this objective. Further, the laser may be any commercially available laser.
The two beams 18, 20 are passed from the circular polarizer 22 to a liquid crystal system 24. The liquid crystal system 24 is in communication with and controlled by a computer system 26, in order to perform phase shifting, as is known. The liquid crystal system 24 is preferably a polarization-based phase shifter.
After the beams 18, 20 pass through the liquid crystal system 24, they are directed to a laser beam expander 28 which expends the two laser beams 18, 20 and allows them to overlap or superimpose in the space to interfere with each other to form a fringe pattern or line array 30 on the surface 32 of the part to be measured.
With particular reference to Figure 2 there is shown an alternative embodiment of the light source 10 for the measurement system.
In Figure 2, the system operates the same as the system shown in Figure 1, except as described below. As shown in Figure 2, the beams 18, 20 exit the birefringent crystal 16 and enter the liquid crystal system 24. In this embodiment, the liquid crystal system 24 is a crystal-based phase retardation phase shifter, which is controlled by the computer system 26 to perform phase shifting. After the beams 18, 20 exit the liquid crystal system 24 they enter the polarizer 22 which orientates the polarization directions of the two beams 18, 20 into the same orientation or direction. The beams 18, 20 then enter the beam expander 28, which projects them on the part 32 in the form of a fringe pattern 30.
Again, while a polarizer is preferably used a variety of other known structures may also be utilized for accomplishing the described function.
With particular reference to Figure 3 there is shown another alternative embodiment of the light source 10 for the measurement system.
In this embodiment, a laser 12 emits a laser beam 14, which is directed to a liquid crystal system 40 and the liquid crystal system 40 is preferably a liquid crystal retardation-based phase shifter.
The liquid crystal system 40 is preferably a combination of a liquid crystal system 24 and a birefringent crystal 16 as described above.
The liquid crystal system 40 can be formed by gluing or otherwise securing the liquid crystal system 24 to the birefringent crystal 16. Alternatively, the liquid crystal system 40 can be formed by combining these structures into a single structure during manufacture.
The liquid crystal system 40 thus splits the laser beam 14 into two separate beams 18, 20, and is controlled by the computer system 26 to effectuate the beam splitting and phase shifting in a single structure.
The beams 18, 20 that exit the liquid crystal system 40 then pass through the polarizer 22, which orientates the polarization directions of the two beams 18, 20, into the same orientation or direction.
The properly oriented beams then pass to the laser beam expander 28 which expands the two beams 18, 20 and allows them to overlap or superimpose in the space and interfere with each other to form a fringe pattern or line array 30 on the surface 32 of the part to be measured. This embodiment results in a more compact system allowing for smaller packaging requirements.
Further, the beams, in this embodiment only, pass through a single structure, which minimizes the potential for diminished light intensity and the single structure also eliminate any alignment problems that may be present in the above described embodiments, which utilize multiple structures.
With particular reference to Figure 4 there is shown a further alternative embodiment of the light source 10 for use with the measurement system.
As shown in Figure 4, a laser 12 emits a laser beam 14, which is directed to a beam expander 42. The beam expander 42 expands the beam 14 into multiple beams 44, 46, which converge and intersect at a point 48 before entering a liquid crystal system 50.
The beam expander 42 thus illuminates the liquid crystal system 50. In this embodiment, the liquid crystal system 50 is a spatial light modulator system having two slots, 54, 56, or two circular holes.
The liquid crystal system 50 is in communication with a computer system 52, which performs phase shifting on the beams 44, 46. The number of slots or holes may obviously be varied by the computer system 52.
Additionally, the distance 'd' between the holes or slots may also be varied by the computer system 52. The liquid crystal system 50 emits two beams 58, 60, which will interfere in space to generate a fringe pattern 62 on a part surface 64 to be measured.
After the fringe pattern 30 is projected onto the part surface 32, an image is formed that includes the fringe pattern and the part surface being measured. The image formation on the part surface 32 preferably employs diffraction effect, as is well known in the art, so that the fringe pattern exists in space and effectively eliminates any defocus problem. Other known image formation techniques may be utilized.
Multiple fringe patterns are preferably formed on the surface of the part 32 to be measured. In the preferred embodiment, three fringe patterns are generated on the part surface in order to ensure accurate measurement of the part surface geometry. However, it should be understood that four or more patterns may be generated.
After the first image has been formed, a camera or other photographic device (not shown) takes a picture of the image which is comprised of the fringe pattern on the part surface.
The computer 26 then signals the liquid crystal system 24 in order to phase shift the fringe pattern, as is well known in the art. The phase shifting assists in the accurate measurement of the part surface 32.
Thereafter, another photograph is taken of this subsequent image resulting from the phase shift. Phase shifting is then preferably performed again in order to generate a third image. A photograph is then taken of this third image. Preferably this process is performed three times but it may be performed more or less times in accordance with the present invention.

Claims

A fringe generator for use in connection with a measurement system for measuring the surface geometry of a part (32), comprising a laser (12) for emitting a laser beam a transparent device (16) disposed forwardly of said laser for splitting said laser beam into a plurality of beams, a liquid crystal system (24) disposed forwardly of said transparent device (16), a computer (26) in communication with said liquid crystal system (24) to perform phase shifting on said plurality of beams and a beam expander (28) for expanding said pair of beams to form a fringe pattern (30) on the part (32).
A measurement system as claimed in claim 1 wherein said liquid crystal system is a liquid crystal polarization based phase shifter (24).
A measurement system as claimed in claim 1 or in claim 2 further comprising a polarizer (22) which is disposed between said transparent device (16) and said liquid crystal system (24).
A measurement system as claimed in claim 1 wherein said liquid crystal system is a liquid crystal based phase retardation system (24).
A measurement system as claimed in claim 1 or in claim 4 further comprising a polarizer (22) which is disposed between said liquid crystal system (24) and said beam expander (28).
A measurement system as claimed in claim 1 or in claim 4 or in claim 5 wherein said transparent device and said liquid crystal system are combined into a single structure (40).
A measurement system as claimed in claim 1 wherein said liquid crystal system is a spatial light modulator (50).
A measurement system as claimed in any preceding claim wherein said transparent device is a birefringent crystal (16).
A method for generating a fringe on a part surface, which is to be measured, characterised in that the method comprises emitting a beam from a light source (12), splitting said beam into a plurality of beams, phase shifting said plurality of beams and expanding said pair of beams such that they overlap in space to interfere with each other to form a fringe pattern (30) on the surface of the part (32).
A method as claimed in claim 9 further comprising orienting the polarization directions of each of said plurality of beams in the same direction.
A method as claimed in claim 10 wherein said orienting is performed before said phase shifting of said plurality of beams.
A method as claimed in claim 10 wherein said orienting is performed after said phase shifting of said pair of beams.
A method as claimed in any of claims 9 to 12 wherein said phase shifting is controlled by a computer system (26).
A method as claimed in any of claims 9 to 13 wherein said splitting and said phase shifting are performed by a single structure (40).
A method for accurately measuring without contact the surface of a vehicle body part characterised in that the method comprises a method as claimed in any of claims 9 to 14.